1. Field of the invention:
This invention relates to a semiconductor laser device having a thin active layer for laser oscillation, the thickness of which is carefully controlled in a liquid phase epitaxial growth process, thereby attaining a high output power operation.
2. Description of the prior art:
In order to achieve continuously stable laser oscillation in a single mode at a low threshold current level, semiconductor laser devices have been designed in a manner to define the active layer by cladding layers, and moreover to confine light and carriers with a high density within the laser oscillation area of the active layer by means of a current-confining striped structure and a waveguide structure. However, when the semiconductor laser devices operate at a high output power, the density of light in the active layer is increased, resulting in heat deterioration of the devices. To resolve this problem, TRS (a high-power, single-mode laser with twin-ridge-substrate structure) lasers have been proposed (Appl. Phys. Lett., vol. 42, No. 10, 15th May, 1983, p. 853) in which a thin active layer is formed to decrease the light density of the active layer thereby attaining high output power operation. The thin active layer is grown on the ridges of the base layer by liquid phase epitaxy. However, the growth of an active layer having a thickness of as thin as 0.1 .mu.m or less using liquid phase epitaxy is extremely difficult. The output intensity of laser light depends upon the thickness of the active layer, and as the thickness of the active layer is decreased, the carrier density of the active layer increases resulting in a state for laser oscillation at a high output power at a low current level. However, even though a thin active layer is grown on the ridges of the base layer and accordingly a decrease in the light density thereof is achieved, the absorption of laser light at or around the facets does not decrease, causing damage and/or deterioration of the facets, which shortens the life time of these TRS lasers.
On the other hand, window VSIS lasers have been proposed (Appl. Phys. Lett., vol. 42, No. 5, 1st May, 1983, p. 406) in which a thick and concaved active layer is used in the stimulated region inside of the facets and a thin and flat active layer is used in the window regions in the vicinity of the facets to thereby decrease the absorption of laser light at or near the facets, so that a high output power operation can be attained. However, so long as the growth of the active layer is carried out using a common liquid phase epitaxial growth technique, the curvature of the active layer in the light stimulated region cannot be controlled within a fixed limit due to problems such as the saturation of the Ga solution and/or of the temperature distribution within the growth furnace. Thus, semiconductor laser devices having uniform characteristics cannot be mass-produced with reliability. Moreover, the application of such a concaved active layer to the devices results in an increase in part in the light density of the concaved active layer with relation to the refractive index of the active layer, causing deterioration of the devices at high output power operation.